When I delete the first post of the thread all rest also being deleted any way to prevent this.
Well this is the default of mybb
Read the message u get when deleting a thread
regards
Go to your Admin-CP -> Users and Groups -> Manage Groups -> *Usergroup* -> Edit Usergroup -> Can Delete Own Threads -> No.
Even if the first post deleted why can not others stay why do they have to go also.
Usually the first post is introducing the main topic of the thread. Without it, the thread is usually incomprehensible. (for example, if you delete the first message in this thread, all these replies may not make sense). You can try to split the first post into a new thread, then delete that (maybe that works)